LT3475IFE datasheet(13/20 Pages) LINER

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LT3475/LT3475-1

3475fb

Table 4. Schottky Diodes

(V)

IAVE(A)

(A)

VF at 1A

(mV)

VF at 2A

(mV)

On Semiconductor

MBR0540

0.5

620

MBRM120E

530

MBRM140

550

Diodes Inc

B120

500

B130

500

B140HB

530

DFLS140

1.1

510

B240

500

International Rectiﬁer

10BQ030

420

BOOST Pin Considerations

The capacitor and diode tied to the BOOST pin gener-

ate a voltage that is higher than the input voltage. In

most cases, a 0.22μF capacitor and fast switching diode

(such as the CMDSH-3 or MMSD914LT1) will work well.

Figure 5 shows three ways to arrange the boost circuit.

The BOOST pin must be more than 2.5V above the SW

pin for full efﬁciency. For outputs of 3.3V and higher, the

standard circuit (Figure 5a) is best. For outputs between

2.8V and 3.3V, use a small Schottky diode (such as the

BAT-54). For lower output voltages, the boost diode can be

tied to the input (Figure 5b). The circuit in Figure 5a is more

efﬁcient because the BOOST pin current comes from a

lower voltage source. The anode of the boost diode can

be tied to another source that is at least 3V. For example, if

you are generating a 3.3V output, and the 3.3V output is on

whenever the LED is on, the BOOST pin can be

connected to the 3.3V output. For LT3475-1 applications

with higher output voltages, an additional Zener diode

may be necessary (Figure 5d) to maintain pin voltage

below the absolute maximum. In any case, be sure that

the maximum voltage at the BOOST pin is both less than

60V and the voltage difference between the BOOST and

SW pins is less than 30V.

The minimum operating voltage of an LT3475 application

is limited by the undervoltage lockout (~3.7V) and by the

maximum duty cycle. The boost circuit also limits the

minimum input voltage for proper start up. If the input

voltage ramps slowly, or the LT3475 turns on when the

output is already in regulation, the boost capacitor may

not be fully charged. Because the boost capacitor charges

Figure 5. Generating the Boost Voltage

VIN

BOOST

GND

VIN

LT3475

(5a)

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

VIN

BOOST

GND

VIN

LT3475

(5b)

VOUT

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

VIN

BOOST

GND

VIN

LT3475

(5c)

3475 F05

VOUT

VBOOST – VSW ≅ VIN2

MAX VBOOST ≅ VIN2 + VIN

MINIMUM VALUE FOR VIN2 = 3V

VIN2 > 3V

VIN

BOOST

GND

VIN

LT3475

(5d)

3475 F05

VOUT

VBOOST – VSW – VZ

MAX VBOOST ≅ VIN + VOUT – VZ

APPLICATIONS INFORMATION

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